Protective arrangement for electrical instruments



y 1937. H. w. BOUSNIAN 2,079,485

PROTECTIVE ARRANGEMENT FOR ELECTRICAL INSTRUMENTS Filed Oct. 25, 1934 Fig.1.

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Inventor":

Henr W Bousman,

Att, or ne s.

Patented May 4, 1937 UNITED STATES PROTECTIVE ARRANGEMENT FOR ELEC- TRICAL INSTRUMENTS Henry W. Bonaman, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application October 23, 1934, Serial No. 749,543

lclaim.

My invention relates primarily to a scheme for protecting electrical devices such as measuring instruments, relays and the like, against excessive voltages, and it is a primary object of my invention to provide such protection yet without interfering with the operation of the instrument or relay which is protected when such excessive Voltages occur. The invention is particularly suitable for use in connection with instruments used in null measurement schemes where high sensitivity is desired at very small detecting voltages and where it is desired that the sensitivity of the instrument shall decrease, but yet give an indication at higher voltages.

In carrying my invention into effect in its preierred form, I connect a storage battery or the equivalent and a rectifier in series relation in shunt to the instrument circuit to be protected. For voltages below that of the storage battery the shunt circuit has no effect since the rectifier is connected in such direction that the battery cannot discharge through the instrument circuit. Consequently until the applied voltage becomes suiiicient to charge the battery, a current proportional to the first power of such voltage is applied to the instrument. However, as the voltage increases, the battery starts to charge and the current applied to the instrument no longer increases in a linear relation with the applied voltage.

The scheme is suitable with slight modification for either direct or alternating current systems.

The features of my invention which are believed to be novel and patentable will be pointed out in the claim appended hereto. For a better understanding of my invention, reference is made in the following description to the accompanying drawing, in which Fig. 1 represents my invention applied to the detecting instrument of a null bridge measuring system. Figs. 2 and 3, respectively, show the character of scale calibrations for an instrument before and after applying my invention thereto- Fig. 4 represents the invention where the sensitivity of the protective shunt may which are immaterial and are not indicated. The

bridge is shown as supplied from a hlghirequency source represented at I i and it is assumed. that the bridge is used in some measuring scheme so as to obtain zero voltage across the detecting terminals I! when the bridge is finally balanced in a measuring operation. This is referred to generally as a null method because in the operation of the bridge to obtain a balance the detecting instrument which is usually connected across the detecting terminals 12 comes to a zero indication when a balance is obtained and the actual measurement reading is otherwise obtained as, for example, by the position of an adjustable device in one arm of the bridge. My invention is particularly beneficial for protecting the detecting instrument of such a bridge because in the initial stages of obtaining a bridge balance the voltage across terminals iz may be considerable, whereas the instrument used should be one sensitive to very low voltages.

In Fig. 1 the detecting instrument is represented at l3 and the protective arrangement of my invention comprises a battery ll connected across the detecting terminals I! in shunt to instrument l3 through rectifiers i5 so arranged with respect to the polarity of battery N that the battery cannot discharge through these rectifiers but may be charged by full wave rectification in case the voltage across the detecting terminals l2, when rectified, is high enough to overcome the counter E. M. F. of the battery. The detecting instrument I3 is represented as of the direct current rectifier type and is connected to the detecting terminals i2 through a filter indicated at IS. The instrument I 3 may be considered to be a sensitive voltmeter or millivoltmeter which without my protective scheme applied to the circuit would have substantially a linear scale calibration such as is represented in Fig. 2, the instrument being callbrated with the filter to represent a voltage proportional to that across the detecting terminals it. Such an unprotected arrangement is sensitive but if, as is likely to happen in the initial stages of a bridge measuring operation, a voltage corresponding to several times the normal full scale range of the detecting instrument should appear across terminals l2 instrument I! would likely be seriously damaged. a

With my protective scheme connected across the detecting circuit as represented the instrument l3 may then be recalibrated as represented in Fig. 3.

Up to about five volt units the instrument It is Just as sensitive as before so that it has the desirable characteristics necessary for accurately balancing the bridge [0. However, for higher voltages current now begins to flow into battery i4 through the rectifiers i5 so that the range of instrument I3 is not reached until fifty-five volt units appear across terminals i2 yet instrument i 3 indicates and guides the operator in balancing of the bridge and as a balance is approached and the voltage across terminals [2 approaches zero the protective device becomes inactive and the instrument Hi again operates over the more sensitive portion of its calibration range. The voltage of the battery will be so proportioned with the voltage drop through the filter and other conditions of the circuit as to produce the results desired. Preferably the battery will have a current capacity sufilciently large as compared to the small current that flows in the detector cirsuit that its counter E. M. F. will remain substantially constant after considerable use. However, it may be noted that a change in instrument calibration due to any expected rise in battery voltage is immaterial in any measuring system in which the detecting instrument reads zero at the time of measurement.

Fig. 4 shows the protective scheme between an alternating current source I! and an instrument or other electrical device 18 where an adjustable potential divider I9 is connected across the battery oi the biased rectifier protective device. By adjustment of the potential divider the voltage of source ll at which the protective device starts to become eifective may be varied. Also the potential divider resistance allows the battery l4 to discharge slowly with low voltages of source [1 thereby tending to maintain the battery voltage substantially constant and the system in calibration with alternate high and low voltages of source ll.

Substantially the same results of the rectifier bridge of Fig. 1 may be obtained with two batteries 20 and 2| connected in series with rectifiers 22 and 23 across the circuit of the device 18 to be protected as represented in Fig. 5. Battery 2!! and rectifier 22 are connected in reverse to battery 2! and rectifier 23 in shunt to the device 58. The rectifiers and batteries are, of course, connected as in the other arrangements so that the batteries will not discharge through the rectifiers but may be charged when their counter E. M. F. is exceeded. An adjustable impedance 24 may be used in series with device I 8 to assist in calibration or to change the calibration as desired. While the arrangements of Figs. 4 and 5 may be used with either direct or alternating current circuits it is unnecessary to use a full wave rectifier arrangement with direct current and for direct current circuits where the polarity does not change the biased rectifier may be simplified as represented in Fig. 6.

The effect or changing the impedance 24 of Fig. 5 is represented in the curves of Fig. 7 where the voltage of source I! represented by V is plotted against the current I in the device It. For a given value of impedance at 24 the current through the device i8 and consequently its defiection characteristics, it it be an indicating instrument, may be represented by curve A. At a voltage corresponding to the value N the protective scheme comes into operation. It the impedance at 24 be increased the current through device i8 will be less as represented by the curve B. Without the protection the current in device l8 would increase as per dotted line curve C with an increase in voltage.

It is seen that I have provided a simple, easily adjustable way of protecting electrical devices against overvoltages but without the necessity of cutting such devices out of service or interiering with their operation when the protective feature comes into operation.

What I claim as new and desire to secure by Letters Patent of the United States is:

A voltage protective system comprising in combination a measurement bridge having terminals across which a voltage proportional to the condition of unbalance of said bridge appears when the bridge is in use, a sensitive electrical instrument subject to such voltage to guide in balancing the bridge, and means including a rectifier and a storage battery in series relation connected in shunt relation to said terminals such that the battery cannot discharge through the rectifier but may be charged therethrough, the battery having a counter E. M. F. such as to prevent charging when the bridge is substantially balanced and the voltage applied to said instrument is low but such as to cause charging when the bridge is appreciably unbalanced and the voltage across said terminals tends to be in excess of that which it is desirable to apply to the instrument.

HENRY W. BOUSMAN. 

